diff --git a/src/matrix/ssss/common.ts b/src/matrix/ssss/common.ts
index c4d0adeb..b7c06192 100644
--- a/src/matrix/ssss/common.ts
+++ b/src/matrix/ssss/common.ts
@@ -22,6 +22,7 @@ export interface IKeyDescription {
         algorithm: string;
         iterations: number;
         salt: string;
+        bits?: number;
     };
     mac: string;
     iv: string;
diff --git a/src/matrix/ssss/index.ts b/src/matrix/ssss/index.ts
index f0c72e0f..15e95200 100644
--- a/src/matrix/ssss/index.ts
+++ b/src/matrix/ssss/index.ts
@@ -15,7 +15,7 @@ limitations under the License.
 */
 
 import {KeyDescription, Key} from "./common";
-import {keyFromPassphrase} from "./passphrase.js";
+import {keyFromPassphrase} from "./passphrase";
 import {keyFromRecoveryKey} from "./recoveryKey.js";
 import {SESSION_E2EE_KEY_PREFIX} from "../e2ee/common.js";
 import type {Storage} from "../storage/idb/Storage";
diff --git a/src/matrix/ssss/passphrase.js b/src/matrix/ssss/passphrase.ts
similarity index 86%
rename from src/matrix/ssss/passphrase.js
rename to src/matrix/ssss/passphrase.ts
index 9460e4db..00e4801e 100644
--- a/src/matrix/ssss/passphrase.js
+++ b/src/matrix/ssss/passphrase.ts
@@ -15,6 +15,8 @@ limitations under the License.
 */
 
 import {Key} from "./common";
+import type {KeyDescription} from "./common";
+import type {Platform} from "../../platform/web/Platform.js";
 
 const DEFAULT_ITERATIONS = 500000;
 const DEFAULT_BITSIZE = 256;
@@ -25,7 +27,7 @@ const DEFAULT_BITSIZE = 256;
  * @param  {Platform} platform
  * @return {Key}
  */
-export async function keyFromPassphrase(keyDescription, passphrase, platform) {
+export async function keyFromPassphrase(keyDescription: KeyDescription, passphrase: string, platform: Platform): Promise<Key> {
     const {passphraseParams} = keyDescription;
     if (!passphraseParams) {
         throw new Error("not a passphrase key");